X-ray-ted surprise in a local galactic cluster

New images from the Chandra X-ray observatory reveal a surprise event in an …

In a nearby galactic cluster, there is more than meets the eye. When galaxy 3C438 is viewed in the optical spectrum (top image) it looks like your normal, everyday, run-of-the-mill collection of stars. However, beneath the peaceful facade viewed in the optical spectrum—in the X-ray region of the spectrum—lies a violent explosion of hot gas spanning over 2 million light years (middle image). A closer examination of the center of the explosion using radio telescopes (the VLA) shows two jets of gas moving away from the center of the explosion (inset, bottom image). The galaxy at the heart of this phenomenon, 3C438, is near the center of a massive galactic cluster that contains gas over 170 million degrees Celsius in temperature. "The huge feature detected in the cluster, combined with the high temperature, points to an exceptionally dramatic event in the nearby Universe," said Ralph Kraft of the Harvard-Smithsonian Center for Astrophysics in Cambridge, MA.

These findings were recently presented at the annual American Astronomical Society conference and will appear in an upcoming issue of the Journal of Astrophysics. Two explanations have been put forth to explain this massive explosion: a cosmic collision and a supermassive black hole. The collision theory postulates that two galactic clusters are colliding with one another at a rate of nearly 4 million miles per hour. The theory suggests that the collision is producing a massive shock wave that leads to a huge pressure drop across the wave front, which creates the arc that is being seen today. While this theory is the most popular, it has a shortcoming: there is only one peak in the measured X-ray emission spectrum. If two bodies were colliding, you would expect to see two distinct peaks, not the one that is observed.

The other explanation is that matter has been falling into a supermassive black hole at the center of the galaxy. While black holes suck in nearby matter and light, they can also expel a portion of it as jets of matter and light that shoot from them at high speeds. It is thought that these jets could be heating and pushing the interstellar gas, which would explain the current observations. This hypothesis also has a flaw. While it is believed that there is a supermassive black hole at the center of 3C438, in order to account for the display seen the black hole would need to have swallowed about 30 billion solar masses over the past 200 million years—a rate that is physically implausible.

Further observations of the phenomenon will be made both by Chandra and the XMM-Newton X-ray observatory in the hope that they can help discriminate between these two options. Whatever the source of the explosion is, be it collision or black hole, it will easily be the most massive of either ever viewed by humans.

Matt Ford / Matt is a contributing writer at Ars Technica, focusing on physics, astronomy, chemistry, mathematics, and engineering. When he's not writing, he works on realtime models of large-scale engineering systems.